This invention relates generally to a semiconductor integrated circuit device having a piezoelectric type oscillator, such as, a crystal or ceramic oscillator, which have a high stability in terms of oscillation frequency and more particularly pertains to a semiconductor integrated circuit device having a piezoelectric type oscillator which properly drives the piezoelectric oscillator and internal circuitry through the application of a voltage lower than the voltage required for oscillation of the piezoelectric type oscillator.
In the field of semiconductor integrated circuits, clock signal generators of the type having a piezoelectric type oscillator, such as, a crystal or ceramic oscillator, have been widely utilized due to their superior stability in terms of oscillation frequency. For example, crystal oscillator circuits generally exhibit of frequency stability of 10.sup.-5 or greater. While CR oscillator circuits have also been widely utilized, they generally exhibit lower freqeuncy stability. Therefore, sophisticated semiconductor circuits, such as, microprocesors, are usually employed together with the piezoelectric type oscillator.
A typical 4 bit microprocessor semiconductor integrated circuit unit is illustrated in FIG. 10 and comprises a crystal oscillator circuit 1 provided with an external crystal resonator 1A having a characteristic frequency of approximately 32.768 KHz and an internal circuit 2 comprising a centrl processing unit and the like. Internal circuit 2 utilizes clock signals generated by crystal osicllator circuit 1 as internal system clock signals. Circuit 2 is provided with current from external power source 3 through a power switch SW. Power source 3 must have a voltage level sufficiently high to permit crystal oscillator circuit 1 to properly oscillate. As an example, crystal oscillator circuit 1 is driven to oscillate with a voltage of about 1.1 V. Thus, external power source 3 must have a voltage level of 1.1 V or higher. In addition, the internal circuits in semiconductor integrated circuit 2 must be provided with a sufficiently high voltage for proper operation, which is usually higher than 1.1 V.
An object of this invention is to provide a semiconductor integrated circuit having an oscillator circuit wherein the semiconductor integrated circuit can be operated with an applied voltage lower than the voltage required for operating the oscillator circuit.